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Medical prosthetic devices presenting enhanced biocompatibility and wear resistance, based on colbalt alloys and process for their preparation

Medical prosthetic devices presenting enhanced biocompatibility and wear resistance, based on colbalt alloys and process for their preparation description/claims

The present invention concerns medical prosthetic devices, or medical implants, consisting of cobalt or cobalt alloy, presenting enhanced characteristics of biocompatibility, hardness and wear resistance, and it refers to a process for their preparation.

In the prosthetic field, the surfaces coupling in the movement of an artificial articular joint (hip and knee joints) are in direct contact, presenting significant friction and wear. Co—Cr—Mo—Ni alloys are widely used for articular prostheses, in particular in total hip joints, with a metal-on-metal-contact. They produce, inside the artificial joint, a moderate, but measurable amount of wear debris.

It must be considered that this debris presents potentially a high toxicity, in fact it can release Cr, Co and Ni as metal ions presenting different oxidation grade, being in contact for a long time with physiological fluids.

Some data reported in literature underline the potential danger ability of metallic wear debris, even if in reduced amount (G. Gasparini et al.[1]). They arouse a typical reaction in the organism. A homogeneous population of cells is formed and they are activated by phagocitosis, because of the reduced dimensions (20-100 nm) of metallic wear debris. They can release factors which induce osteolitic phenomena through the activation of osteoclasts.

Furthermore frequent cases of cellular necrosis are revealed when wear debris production is especially high during a reduced time and they can enhance osteolitic phenomena.

Certainly the released ions interfere with the cellular metabolism and they cause toxic and mutagen biochemical reactions, in some cases also of the immune system, up to vascular damage with bony necrosis and the consequent prosthetic failure. The eventual correlation between the presence of metallic ions and the frequency of malignant tumours is still dubious (P. Rossi et al.[2]).

The haematic and urinary concentrations of Co—Cr—Ni—Mo in thirty patients presenting a hip joint were analysed in a recent paper written by Massè et al.[3]. The work underlines a relevant increment in the concentration of Co and Cr in patients with a metal-on-metal prosthesis.

Considering the composition, niobium and tantalum are promising elements in order to obtain a highly compatible surface (D. M. Findlay et al.[4]). In fact they show an exceptional corrosion resistance and, in particular, tantalum was recently used with success for osteointegrated devices in the prosthetic field.

In fact it can be found on the marketing several devices made out of porous tantalum, containing 99 wt % of tantalum and 1 wt % of amorphous carbon, used as acetabular caps or as scaffolds for hip reconstruction when a relevant amount of bone was lost (X. Zou et al.[5]).

Some investigations showed that tantalum do not induce cytotoxic phenomena and that osteoblastic cells adhere, proliferate and differentiate easily on it (D. M. Findlay et al.[6]).

Other in-vivo investigations on animals showed that there is no trace of metal, as metallic ions, in animal tissues around the implant, confirming its high corrosion resistance and low metal ion release (H. Matsuno et al.[7]).

EP-A-0 555 033 describes medical implants made out of a titanium, zirconium or cobalt alloy, containing up to 2 wt % of a easily oxidisable or nitridable metallic solute, as tantalum, and presenting a surface layer of the implant, where this solute is oxidised or nitridated through internal oxidation or nitring, with the aim of inducing an enhanced surface hardness and abrasion resistance.

The described process so requires the use of modified bulk metal alloys to include said solute.

The present invention supplies prosthetic devices or medical implants made out of conventional cobalt alloys on the marketing and showing enhanced biocompatible characteristics, low metal ion release, higher hardness and good tribological behaviour, because of a surface modification treatment that induces the formation of intermetallic compounds and the formation of a surface diffusion layer of tantalum and/or niobium on the alloy.

WO02/068007 and WO02/068729 describe surface modified biomedical implants with tantalum, by using electrodeposition processes from molten salts or by CVD deposition. However, in this case the surface shows an external layer of Alfa-tantalum presenting high ductility, that is absent in the present invention, that furthermore uses a process in molten Salts without applying any electrical field.

So it is an object of the present invention a medical prosthetic device or a medical implant comprising a bulk made out of cobalt or cobalt alloy, characterised by a thin surface layer, with an enrichment of tantalum and/or niobium in the composition, respect to the composition of the bulk material, and presenting intermetallic compounds which are rich in tantalum.

More characteristics of the devices are described in the included dependent claims.

The preferred process for the production of the prosthetic devices, that is a further object of the invention, includes a treatment of the cobalt or cobalt alloy in a mixture of molten salts, without applying any electrical field, comprising a tantalum and/or niobium halide, eventually added—respectively—with metal tantalum and/or niobium.

This process according to the invention must be preferred, because it has the advantage of inducing a surface modification presenting a strong interface between the surface and the bulk, through the presence of a diffusion layer.

The process of treatment in molten salts is preferably performed by using a mixture containing between 20 wt % and 100 wt % of a tantalum and/or niobium halide and/or by using between 0 wt % and 80 wt % of an alkaline or alkaline earth halide.

The tantalum halide is preferably K2TaF7, but also others salts containing tantalum can be used, such as fluorides (TaF3 [TaF5]4), chlorides (TaCl3, TaCl4, TaCl5), bromides (TaBr3, TaBr4, TaBr5) and iodides (TaI4, TaI5). Analogous compounds can be used in the case of niobium.

The alkaline or alkaline earth halide is preferably sodium chloride, but other chlorides, bromides or iodides of Na, K, Li, Ca e Mg can be used.

Preferably, the bath of molten salts is added by pure tantalum and/or niobium, as metal powder, by using a concentration up to 20 wt % and more preferably lower than 5 wt %, respect to the composition of the mixture of molten salts.

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